Conventional bonding techniques put plates or three-dimensional pressed metallic workpieces on top of each other and bond the metallic workpieces together by electric resistance welding, arc welding, adhesive bonding using an adhesive, or mechanical fastening using bolts or rivets.
When the metallic workpieces have three-dimensional shapes, spot welding capable of locally bonding a plurality of joining parts is used.
A bonding method disclosed in Jpn. Pat. No. 2712838 is another technique that uses friction stir bonding in a nonfused state. This bonding technique drives a rotating projection called a probe into the joint of the adjoining surfaces of two workpieces to bond together the adjoining parts by plasticizing their metallographical structures by frictional heat.
Surface treatment methods of a casting, such as a joining surface to be bonded to the cylinder block of a cylinder head, disclosed in JP-A Nos. 183316/1998 and 15426/2000 rotates a rotating tool having a shoulder and a projection projecting from the shoulder, and presses the rotating tool against the surface to heat the surface in a nonfused state and to stir the nonfused surface of the casting.
Joining parts are bonded incompletely or melt if the rotating speed of the tool is excessively high, the depth of feed of the tool is excessively great or the tool is fed at an excessively high feed rate in bonding the joining parts by friction stir bonding in the nonfused state. Therefore, there is a limit to the reduction of processing time.
The aforesaid known bonding technique determines through experiments beforehand the control parameters, including an optimum rotating speed of the tool suitable for the thickness and materials of workpieces to be bonded together and the depth of feed of the tool. Therefore, experiments must be conducted again to determine optimum control parameters when the design of the workpieces is changed or different workpieces are bonded together. A test process is necessary for the evaluation of quality, such as bond strength, and a sample obtained by actually bonding workpieces together must be subjected to a tensile test or the like.